In electrooptical displays based on liquid crystals, which are also referred to as liquid crystal displays or LCDs, the liquid crystal phase is present as a dielectric between two sheetlike electrodes which are spaced from each other and are constructed in patterns or grids; selective excitation of predetermined electrode field regions has the effect of changing the position or orientation of the molecules of the dielectric within the regions of the selective excitation and thereby of bringing about a visible selective local modification of the dielectric.
This effect requires potentials whose size is dependent inter alia on the degree of anisotropy of the liquid crystal phase; this degree of anisotropy is expressed in general for a given liquid crystal substance by the difference (.DELTA.) of the dielectric constants (.epsilon.) parallel to the electric field (.epsilon..parallel.) minus that perpendicular thereto (.epsilon..perp.), i.e. by the relationship referred to in short as dielectric constant anisotropy or DCA .DELTA..epsilon.=.epsilon..parallel.-.epsilon..perp..
In liquid crystal displays which are based on the principle of the twisted nematic cell (TN=twisted nematic; see W. Helfrich, and M. Schadt, Appl. Phys. Lett. 18 (1971) 127), a property which is dependent upon the DCA is the so-called threshold voltage, which in turn must not be excessively high for various reasons, such as CMOS compatibility, and which in practice can be the smaller, the higher the value of .DELTA..epsilon. is.
Since .DELTA..epsilon. is the difference of two values, the same difference can be the result of very different pairs of values, for example the resulting DCA being the same for EQU .DELTA..epsilon.=21(.epsilon..parallel.)-20(.epsilon..perp.)=1 EQU .DELTA..epsilon.=2(.epsilon..parallel.)-1(.epsilon..perp.)=1.
For a completely different reason, namely because of the matrix addressing, which is essential in particular for displays having high information content (for example within the range of up to 10.sup.6 image dots per display), and the attendant desirability of a very steep and high shoulder of discontinuity in the voltage/contrast diagram of the liquid crystal dielectric of the liquid crystal display, the value of the ratio .DELTA..epsilon./.epsilon..perp. should in addition be small; from the above numerical example it will be readily understood that this ideal can be approached in two ways, namely:
(a) by using appropriate liquid crystal mixtures of substances having a strongly positive or weakly negative DCA (see for example A. R. Kmetz, SID Digest, Tech. Papers IX (1078) 70); this has in particular the disadvantage that the mixtures tend to form smectic phases; PA0 (b) by using substances which, in the molecule, have a marked polarization not only in the longitudinal direction but also in the transverse direction, relative to the generally "stretched" form of the liquid crystal molecules (M. A. Osman, European Patent Application Publication No. 0,019,665); this has the advantage that the ratio .DELTA..epsilon./.epsilon..perp. can be reduced by increasing the contribution and not or not necessarily by reducing the .epsilon..parallel. contribution of the same molecule. The disadvantages of the compounds described in the citeed European patent application reside in their relatively high viscosity or relatively high melting points and in the usually difficult synthesis and/or low stability. PA0 (a) the sum of n and m is at least 1 and at most 3, PA0 (b) m is 2 and n 0 if Z is --CH.sub.2 -- and X.sup.4 nitrile, PA0 (c) at least one of the groups X.sup.2 and X.sup.3 is not hydrogen and PA0 (d) n is 0 or 1 if Z is --CH.sub.2 -- and X.sup.4 halogen. PA0 A is the radical ##STR8## and B is the radical ##STR9## in which R, n, m, X.sup.2, X.sup.3 and X.sup.4 have the abovementioned meaning. ##STR10##